Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications (Darwin, 1859, p. 413f).

Saturday, 11 October 2008

The Evolution Slogan

The term "evolution" can be used recklessly in a variety of ways: "If evolution was outlawed, only outlaws will evolve", "Paraphyly is evolution all the way" (Brummitt, 2002:40) and most recently, "Because we understand how evolution happens, we can also guess where it will go next" (Jones, 2008; see also John Wilkins's post). The two main points of contention, highlighted in the latter statement by Steve Jones, are our "understanding" of evolution and our ability to "guess".

The late Colin Patterson, ichthyologist at the then British Museum, Natural History in London, gave a presentation that questioned the term. The talk, titled "Systematics and Creationism", was given at American Museum of Natural History (Patterson, 2002) in November 1981. There Patterson noted:

Combined with the above slogans we may suggest that 'evolution' is: a process of descent with modification that results in paraphyly problem arises. How do we see this process?

In order to know we need to be able to observe or measure. Paraphyly, for instance, cannot be observed. It exists only when an artificially delineated taxonomic group is discovered to be monophyletic (homologous) - like 'invertebrates' or 'aliens'. Descent with modification is also difficult to see in action. Although we can see genealogy and ontogeny, they do not constitute 'descent with modification', at least not in the way Jones uses the term.

What systematists and biogeographers know is that evidence for evolution is based on retrodictions - that is past 'predictions' or patterns. These patterns are homologies or relationships - evidence for evolution. Our task as systematists is to discover whether our groups are a result of evolution, rather than poor taxonomy. Evolution should not be taken for granted - just because we know it exists doesn't mean we should stop looking. Reptiles, for example, are not an evolutionary group. They are a poorly defined taxonomic group like 'insectivores' and 'creepy, crawling things'. Discovering that taxa within the reptilia share closer relationships with taxa in mammalian than with any other taxon does not validate reptiles as an evolutionary group. The task of herpetologists is find those evolutionary groups and, not to defend existing names that have no evolutionary significance. Patterns, homologies, relationship and monophyletic groups are all the same thing: evidence for evolution.

Now we return to Jones. He, like many other evolutionary biologists, has committed a classic error - assuming that life progresses from an incomplete to complete phase: also known as 'primitive to derived'. A typical example is the 'primitiveness' or 'plesiomomorphy' of Archaeopteryx lithographica. The half bird-half reptile is always considered to be transition - fossilized in the middle of evolving. Like all living things dead or alive, Archaeopteryx is perfect in its own right. It has no hidden agenda, no purpose other than to be Archaeopteryx. If we were to assume, unwittingly and in hindsight, that it was primitive, then we are advocating some purpose or teleology, namely that Archaeopteryx was aiming to become a bird. This sort of thinking gives evolution a bad reputation and opens it up to attack from protagonists of anti-science. The logic behind it does not work. Let us assume for the moment that we could go back in time, back when Archaeopteryx was alive. We would assume, that this is a highly evolved 'reptile', a derived form. See the problem? Archaeopteryx is both derived and primitive at the same time in form and space but not in geological time. The whole 'primitive – derived' argument is based stratigraphic sequence and not evolution (homology).

To counter Jones's argument - we are complete, so is Archaeopteryx and all other life that has ever existed and will ever exist on this planet. What does this completeness say about evolution? Absolutely nothing at all. Instead it tells us of a desire for explanation.

We may think 'nothing in biology makes sense except in the light of evolution' (Dobzhansky, 1973), but without a doubt, evolution only makes sense in the light of homology. Biological classification provides us with the tools to discover relationships and a way to understand the evolution of life. Without it we are just telling never-ending stories. I am sure that in 200 millions years time, an octopod biologist, will wonder how something as incomplete and primitive as Homo sapiens lived for as long as it did.

References

Brummitt, R. K. 2002. How to chop up a tree. Taxon 51: 1-41.Dobzhansky, T. 1973. Nothing in Biology Makes Sense. Except in the Light of Evolution. The American Biology. Teacher, 35:125-129.Jones, S. 2008. Evolution is complete: so where do we go from here? Daily Telegraph Online, http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/10/07/scievolution107.xmlPatterson, C. 2002. Evolutionism and creationism. The Linnean 18: 15-33.